It is well known that many heavy fractions of petroleum crudes, such as atmospheric or vacuum resids (the residual oil remaining after fractional distillation of crude oil to remove lighter components) contain coke precursors and metal compounds in amounts which adversely affect further down-stream processing and also, affect the quality of heavy fuels produced therefrom. Similarly, it is known that bitumens obtained from tar sands and heavy oil deposits are difficult and expensive to process because of their high content of asphaltenes and difficult to remove fine particles of inorganic solids.
The above-mentioned coke precursors include polycyclic hydrocarbons, asphaltenes and the like which tend to break down at elevated temperatures to form carbonaceous materials, often referred to as "coke." In subsequent processing coke may form on the interior walls of refining equipment or be deposited on catalyst to reduce its activity level. Hence, a feed stock with a high coke forming tendency is undesirable. The coke forming tendency of an oil is generally evaluated by the Conradson Carbon method or the Ramsbottom Carbon method. A higher number from such an evaluation indicates a greater tendency for coke deposition on, for example, catalyst when the oil is processed by the fluid catalytic cracking (FCC) process wherein heavy charge-stocks, for example, gas oils are cracked to produce gasoline and other lighter products. In the FCC process, coke is burned from the catalyst in a regenerator to restore catalyst activity and the regenerated catalyst is then recycled for the cracking of additional feed-stock.
The above-mentioned heavy oil charge-stocks often contain compounds of undesirable metals, including nickel and vanadium, which when deposited on FCC catalyst may adversely affect the physical properties of the catalyst and also promote the undesirable production of coke, hydrogen and other light hydrocarbon gases in the operation of the FCC process.
Similarly, the bitumen from tar sands contains minute, sometimes colloidal, particles of sand which, because of the difficulty of removal, cause processing problems in down-stream processing. Also heavy oil deposits often contain fine particles of solids, such as diatomite, which cause similar problems. Although there are vast deposits of such hydrocarbons, their development has been retarded because of the high cost of obtaining and processing synthetic crudes (syncrudes) from such deposits and problems caused by the high content of solids and asphaltenes.
The oil refining industry has long been plagued with the problem of maximizing high value liquid transportation fuels (e.g., gasoline, jet, and diesel fuels) while minimizing the lower value fuel oil, especially residual oil, which is usually high in sulfur and metals. These heavy fuel oils, which are the heavy end of the crude oil, often require further upgrading to decrease the sulfur and metal contents.
In order to produce the feed-stocks for the units in the refinery, continuous distillation is generally used. This comprises an atomspheric crude unit followed by a vacuum unit. Thus, there are two distillation systems, both containing almost the same equipment of a charge heater, exchangers, and a distillation column. Both systems are required because the heavy atmospheric tower bottoms will thermally crack if a vacuum was not applied to the system to permit the separation to take place at a lower temperature. The refining industry is still trying to find ways to upgrade the vacuum bottoms to lighter, more valuable products, but is limited by the equipment employed. This limit is imposed by the time temperature relationship of the feed heaters. Normally one is limited to about 750 degrees F. on the outlet of the heater. Above this temperature thermal cracking will take place. This thermal cracking results in coking of the heater tubes, overloading of the vacuum ejectors, and "unstable" products.
These processing limitations plus the decreasing availability of lighter crudes, are putting pressure on the petroleum refining industry to find acceptable methods to upgrade the vacuum bottoms, as well as tar sand bitumens and heavy oils. There are many technically feasible processes, but the economics are far from optimum. The hydrogen addition processes require high pressures and large volumes of catalyst, which result in high capital investments, high operating costs, and catalyst disposal problems. The carbon rejection processes are basically less capital intensive, but result in degraded products which need to be further treated, and therefore, increase the capital investment. These carbon rejection processes also produce undesirable byproducts such as high sulfur and high metals coke or, if they use a circulating solid, present a large catalyst disposal problem.
Many techniques are known for upgrading such hydrocarbon charge stocks contaminated with the above-described solids and solid-forming contaminants. For example, delayed and fluid coking processes are used. The coking process uses thermal conversion to produce coke and coker gasoline, coker gas oil, etc. The solid coke is usually high in ash and sulfur, and the distillate often must be further treated before it can be used for charging to catalytic cracking or blending. Solvent extraction and deasphalting processes also are used for preparing FCC charge-stocks from resids.
At the present time, the FCC process is considered the "work-horse" of the petroleum refining industry and is used extensively for cracking heavier hydrocarbon charge-stocks to produce lighter, more valuable products, such as gasoline blending stocks. At present, gas oils are the principal charge-stock to the FCC process. The use of residual oils, particularly vacuum resids, as FCC charge-stock is limited due to the high content of asphaltenes and metals. Likewise, the use of FCC charge-stock derived from the tar sand bitumens and heavy oil is also limited for similar reasons.
Historically, the activity of FCC process catalyst has increased from the original sand to the present high activity zeolitic catalysts with zeolite contents of about 25%. Such catalysts are well known in the FCC process art and it is also known that increasing the zeolite content of a catalyst increases its activity for cracking. The use of such zeolitic catalysts has permitted the contact time in the FCC reactor or the FCC riser to be decreased significantly. These changes have allowed the refiner to obtain more throughput, less gas and higher liquid yields.
However, the refiner is coming under increasing pressure to limit the vapor pressure of gasoline, raise the diesel quality, and increase the gasoline octane while converting more and more of the very heavy crude fractions, such as vacuum resids, and the like to higher value products. Therefore, the existing FCC unit's products are becoming a source of concern.
Up to now, it has not been feasible to use zeolitic catalyst with greater than about 30% zeolite content as fresh catalyst addition, because of the reactor design. The typical FCC designs using vertical riser reactors terminating in arms, tees, cyclones or other devices to aid catalyst-oil vapor disengagement require too high a catalyst-oil vapor contact time to effectively use very high zeolitic content cracking catalyst. The typical design contact time is about three seconds in the riser portion of the reactor, which then discharges into a reactor disengaging vessel. Catalyst-oil contact times in this vessel, with superfical velocities of around 3 fps, range from a minimum of 3 to as high as 15 seconds, plus cyclone time. While the catalyst density in this portion of the reactor is usually lower than the catalyst density in the riser portion, the reaction is still proceeding in this location. Much of the reaction is thermal and the overall effect is higher coke levels on spent catalyst, higher gas yields, less liquid yield, poorer quality diesel and bottoms products, and more diolefins. The higher coke levels on spent catalyst results in higher regenerator temperatures, and therefore, lower catalyst to oil ratios.
The current state of the art does not permit a feasible method for achieving short or ultra short contact times in the reactor system. It is typically assumed that the contact times in today's FCC units is 3 seconds or less. However, as discussed above this contemplates only the riser section of the reactor and not the total contact time. Therefore, if a refiner attempts to add significantly more than his normal addition rate of about 1% of the catalyst inventory of 25% zeolite content cracking catalyst as fresh catalyst, he will lose control of his unit. The regenerator temperature will then increase to reduce the catalyst-to-oil ratio, and the reaction will then become more thermal. This will result in more gas and less liquid yield until the regenerator temperature, gas compressor, or gas concentration unit systems are overloaded.
In U.S. Pat. No. 4,263,128, I have disclosed a process for upgrading whole crude and bottoms fractions from distillation of petroleum by high temperature, short time contact with a fluidizable solid of essentially catalytically inert character to deposit high boiling components of the charge stocks on the circulating solid, whereby Conradson Carbon values, salt content and metal content are reduced. Therein, an inert solid, such as particles of kaolin clay, is supplied to a rising column of the charge in a contactor to vaporize most of the charge. Carbonaceous and metallic deposits formed on the particles of circulating solid are burned, after which the solid particles are recycled the contactor.
In U.S. Pat. No. 4,435,272, I have disclosed a process for upgrading such charge-stocks by dispersing the charge introduced into a contactor into a descending curtain of heated particles of an added inert contact material. The charge is vaporized and carbonaceous materials, salt and metals are deposited on the circulating contact material. Deposits on the contact material are then burned off, the heat of combustion is absorbed by the contact material and the heated contact material is recycled to the contactor for vaporizing the charge.
It is also known to spray FCC feed into a riser reactor of a catalytic cracking unit to improve contact between the feed and catalyst.
Such known processes permit increased utilization of the crude (or syncrude) to produce transportation fuels, but they have high capital and operating costs and may create environmental concerns.
Therefore, a primary object of the present invention is a more economic process for producing lower molecular weight, more valuable product, such as liquid transportation fuels, from a higher molecular weight hydrocarbon charge stock contaminated with coke precursors, metal compounds, solids, nitrogen, and the like.
Another object of the present invention is an FCC process permitting the use of very high activity catalyst in an ultra short catalyst-oil contact time system.
A further object is such a process which limits the hydrogen transfer characteristic of the zeolitic catalyst and thereby produces better quality diesel fuel and heavy product (higher hydrogen content), less gas, less secondary cracking and higher catalyst-to-oil ratios due to lower coke formation on the catalyst (lower delta coke).
Yet another object of the invention is an FCC process which permits the use of a zeolitic FCC catalyst with a zeolite content of 40% (by weight) or more of zeolite, and preferably more than 60%, and still more preferably greater than 80%, in order to obtain the conversion necessary for economical operation.
A still further object is such a process allowing higher catalyst-oil contact temperatures which will raise the octane number of the FCC gasoline, while permitting the processing of heavier (higher end point) charge-stocks.
The present invention also permits the processing of multiple hydrocarbon oil charge-stocks, such as atmospheric and vacuum resids, synthetic crudes (syncrudes) from tar sand bitumens and shale oil, thermal virgin naphthas or the like as well as conventional FCC gas oil charge-stocks, to produce more valuable products in a process system which reduces capital and operating costs.
Further, the horizontal contactor reactor used in the process of the present invention allows for actual total contact times of 0.1 to 0.2 seconds in the reaction zone, plus cyclone time, which permits the use of the higher zeolite content fresh catalyst without the negative effects of the current technology. This gives the benefits of the catalytic reactions coupled with control of the thermal reactions in the use of higher reactor temperatures, 1000 to 1100 degrees F., if desired.
Additional objects and advantages of the present invention will be set forth in part in the following description and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalites and combination particularly pointed out in the appended claims.